Pressure Testing Two Stroke Engines

What is pressure testing?

Pressure testing is a test performed on a two stroke engine to ensure that the engine has no air leaks. An air leak in a two stroke engine can cause a lean condition in your engines air-fuel mixture. This in turn can result in serious engine damage. For example: Piston seizure, detonation, etc.

Note: There is absolutely no way to ensure that your engine doesn’t have an air leak without performing the following test.

To pressure test your engine you will need a few special items. See below for instructions on building your own pressure test device. To test the engine: cylinder, head, reed cage, intake manifold and spark plug must all be installed and properly torqued. Exhaust pipe and carb must not be installed.

Here is a pic of a typical Mikuni Pressure Test Kit than can be found at many places on the iternet.

A steel or aluminum plate must be bolted onto the exhaust side of the cylinder using a piece of rubber as a gasket.
A round fitting with pressure gage attached should be clamped into the intake manifold on the cylinder. With equipment in place, use a hand pump to fill the sealed engine with 6-8 lbs of air pressure. Once 6 lbs of pressure is pumped into the engine. Set a timer for 6 minutes. Engine must hold a full 6 lbs of pressure for 6 minutes without any loss. A tight motor should be able to hold this setting for more than 10 minutes without losing any air.

Rule of ThumbThe normal bleed-down pressure loss is 1 psi per minute.

If gauge shows a loss of pressure, squirt all gasket surfaces with a water based soapy solution (Windex, 409, etc.) If there is a leak at any surface you will see soap bubbles. Re-torque any areas that leak.

If you cannot locate the leak by looking externally, consult a qualified two stroke mechanic.

If after 6 minutes the gauge shows no loss in pressure, remove testing equipment and continue assembly of machine.

Building your own pressure testing device. All parts can be found at any good hardware store. total cost should be around $15.

hey guys i was reading through this and something caught my eye... does the rule of thumb (1 psi loss for every minute) only apply after the completed 6lbs for 6min?? should you expect a loss of pressure after the alloted time?

hey guys i was reading through this and something caught my eye... does the rule of thumb (1 psi loss for every minute) only apply after the completed 6lbs for 6min?? should you expect a loss of pressure after the alloted time?

It's capable of getting below 1psi a minute. I've had plenty hold pressure for a very long time. You do have to refill it the first time, to get all the crevices filled with pressure.